THE PHASE-DEPENDENT LINEAR CONDUCTANCE OF A SUPERCONDUCTING QUANTUM POINT-CONTACT

The exact expression for the phase-dependent linear conductance of a w eakly damped superconducting quantum point contact is obtained. The ca lculation is performed by summing the complete perturbative series in the coupling between the electrodes, thus taking into account all poss ible multiple Andreev reflections inside the gap. The failure of any f inite-order perturbative expansion in the limit of small voltage and s mall quasiparticle damping is analysed in detail. In the low-transmiss ion regime this nonperturbative calculation yields a result which is a t variance with standard tunnel theory. Our result exhibits an unusual phase dependence at low temperatures in qualitative agreement with th e available experimental data.