  We have realized nanometer size constrictions in ballistic graphene
nanoribbons grown on sidewalls of SiC mesa structures. The high quality of our
devices allows the observation of a number of electronic quantum interference
phenomena. The transmissions of Fabry-Perot like resonances were probed by
in-situ transport measurements at various temperatures. The energies of the
resonances are determined by the size of the constrictions which can be
controlled precisely using STM lithography. The temperature and size dependence
of the measured conductances are in quantitative agreement with tight-binding
calculations. The fact that these interference effects are visible even at room
temperature makes the reported devices attractive as building blocks for future
carbon based electronics.
