▎ 摘　　要

We investigate the critical current I-C of ballistic Josephson junctions made of encapsulated graphene-boron-nitride heterostructures. We observe a crossover from the short to the long junction regimes as the length of the device increases. In long ballistic junctions, I-C is found to scale as proportional to exp(-k(B)T/delta E). The extracted energies delta E are independent of the carrier density and proportional to the level spacing of the ballistic cavity. As T -> 0 the critical current of a long (or short) junction saturates at a level determined by the product of dE (or Delta) and the number of the junction's transversal modes.