Universal limit of critical-current fluctuations in mesoscopic Josephson junctions

The sample-to-sample fluctuations in the critical current of a disordered Josesphson junction are analyzed by means of a transmission-matrix formalism. If the junction becomes small compared to the superconducting coherence length, the fluctuations at T=0 become of order e${\mathrm{\Delta }}_0$/ħ, dependent only on the energy gap ${\mathrm{\Delta }}_0$ of the bulk superconductors and independent of junction length or mean free path. This universal limit is reached in weak links formed from point contacts or microbridges.