Andrea F. Young and Philip Kim report 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 analyzed to confirm that p-n junctions have a collimating effect on ballistically transmitted carriers. The phase shift observed in the conductance fringes at low magnetic fields is a signature of perfect transmission of carriers normally incident on the junctions, providing direct experimental evidence for "Klein Tunneling." The study demonstrates the potential of graphene heterojunctions for studying relativistic electron tunneling and quantum interference phenomena.Andrea F. Young and Philip Kim report 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 analyzed to confirm that p-n junctions have a collimating effect on ballistically transmitted carriers. The phase shift observed in the conductance fringes at low magnetic fields is a signature of perfect transmission of carriers normally incident on the junctions, providing direct experimental evidence for "Klein Tunneling." The study demonstrates the potential of graphene heterojunctions for studying relativistic electron tunneling and quantum interference phenomena.