Observation of mesoscopic conductance fluctuations in YBa2Cu3O7-delta grain boundary Josephson junctions

Magnetofluctuations of the normal-state resistance RN have been reproducibly observed in high-critical- temperature superconductor ?HTS? grain boundary junctions at low temperatures. We attribute them to meso- scopic transport in narrow channels across the grain boundary line. The Thouless energy appears to be the relevant energy scale. Our findings have significant implications for quasiparticle relaxation and coherent transport in HTS grain boundaries.