The chapter discusses the mean free path of electrons in metals, starting with the historical development of the Drude-Lorentz theory, which postulated free electrons to explain metallic conductivity. However, this theory faced several challenges, particularly in explaining why conduction electrons do not significantly contribute to specific heat. The advent of quantum mechanics resolved these issues, with Pauli and Sommerfeld applying Fermi-Dirac statistics to free electrons in metals, reconciling many contradictions. The chapter then delves into the quantum-mechanical theory, which provides a detailed analysis of electron motion in a crystal lattice. It introduces the concepts of the "number of free electrons" and the "mean free path," which are crucial for understanding metallic conductivity. The mean free path is influenced by temperature and varies with temperature, becoming longer at higher temperatures due to thermal vibrations. The text also covers the Sommerfeld theory, which treats electrons as quasi-free, with their energy proportional to the square of their velocity. This theory is valid for monovalent metals where electrons occupy a single energy band. For multivalent metals, the model is semi-quantitative but less accurate. The chapter discusses experimental methods to determine the mean free path, including measurements in thin films and wires. These experiments are crucial for understanding size effects and the behavior of electrons in magnetic fields. The results from these experiments are compared with theoretical predictions, providing insights into the electronic structure of metals. Finally, the chapter explores magnetic effects in thin conductors, focusing on the behavior of electrons in the presence of longitudinal and transverse magnetic fields. The analysis is particularly detailed for thin films and wires, with experimental data available for wires. The chapter concludes by highlighting the importance of these studies in understanding the complex behavior of electrons in metals.The chapter discusses the mean free path of electrons in metals, starting with the historical development of the Drude-Lorentz theory, which postulated free electrons to explain metallic conductivity. However, this theory faced several challenges, particularly in explaining why conduction electrons do not significantly contribute to specific heat. The advent of quantum mechanics resolved these issues, with Pauli and Sommerfeld applying Fermi-Dirac statistics to free electrons in metals, reconciling many contradictions. The chapter then delves into the quantum-mechanical theory, which provides a detailed analysis of electron motion in a crystal lattice. It introduces the concepts of the "number of free electrons" and the "mean free path," which are crucial for understanding metallic conductivity. The mean free path is influenced by temperature and varies with temperature, becoming longer at higher temperatures due to thermal vibrations. The text also covers the Sommerfeld theory, which treats electrons as quasi-free, with their energy proportional to the square of their velocity. This theory is valid for monovalent metals where electrons occupy a single energy band. For multivalent metals, the model is semi-quantitative but less accurate. The chapter discusses experimental methods to determine the mean free path, including measurements in thin films and wires. These experiments are crucial for understanding size effects and the behavior of electrons in magnetic fields. The results from these experiments are compared with theoretical predictions, providing insights into the electronic structure of metals. Finally, the chapter explores magnetic effects in thin conductors, focusing on the behavior of electrons in the presence of longitudinal and transverse magnetic fields. The analysis is particularly detailed for thin films and wires, with experimental data available for wires. The chapter concludes by highlighting the importance of these studies in understanding the complex behavior of electrons in metals.