This study reports the observation of quantum conductance oscillations in extremely narrow graphene heterostructures, where a resonant cavity is formed between two electrostatically created bipolar junctions. The oscillations are attributed to quantum interference effects and confirm the presence of "Klein tunneling," a phenomenon where carriers transmit through a potential barrier without backscattering. The research demonstrates that graphene provides an ideal medium for quantum engineering of electron wave functions due to its gapless spectrum and ability to create regions of positive and negative doping. The study shows that the conductance oscillations are influenced by the density of carriers in the locally gated region (LGR) and the electric field at the junctions. The oscillations are primarily a function of the density in the LGR and are affected by the application of an external magnetic field, which shifts the phase of the oscillations and connects them to high-field Shubnikov-de Haas oscillations. The study also confirms that the observed oscillatory conductance results from quantum interference phenomena in the graphene heterojunction, with the phase shift at a critical magnetic field indicating perfect transmission at normal incidence. The research highlights the potential of graphene heterojunctions for studying relativistic quantum mechanics and offers insights into the behavior of chiral quasiparticles in graphene. The findings are supported by theoretical models and simulations, and the results demonstrate the importance of nonlinear screening in determining transport through graphene p-n junctions. The study also addresses the temperature dependence of quantum coherence effects and the role of disorder in affecting the conductance oscillations. Overall, the research provides a comprehensive understanding of quantum interference and Klein tunneling in graphene heterojunctions, with implications for the development of quantum devices.This study reports the observation of quantum conductance oscillations in extremely narrow graphene heterostructures, where a resonant cavity is formed between two electrostatically created bipolar junctions. The oscillations are attributed to quantum interference effects and confirm the presence of "Klein tunneling," a phenomenon where carriers transmit through a potential barrier without backscattering. The research demonstrates that graphene provides an ideal medium for quantum engineering of electron wave functions due to its gapless spectrum and ability to create regions of positive and negative doping. The study shows that the conductance oscillations are influenced by the density of carriers in the locally gated region (LGR) and the electric field at the junctions. The oscillations are primarily a function of the density in the LGR and are affected by the application of an external magnetic field, which shifts the phase of the oscillations and connects them to high-field Shubnikov-de Haas oscillations. The study also confirms that the observed oscillatory conductance results from quantum interference phenomena in the graphene heterojunction, with the phase shift at a critical magnetic field indicating perfect transmission at normal incidence. The research highlights the potential of graphene heterojunctions for studying relativistic quantum mechanics and offers insights into the behavior of chiral quasiparticles in graphene. The findings are supported by theoretical models and simulations, and the results demonstrate the importance of nonlinear screening in determining transport through graphene p-n junctions. The study also addresses the temperature dependence of quantum coherence effects and the role of disorder in affecting the conductance oscillations. Overall, the research provides a comprehensive understanding of quantum interference and Klein tunneling in graphene heterojunctions, with implications for the development of quantum devices.