MICROSCOPIC THEORY OF THE PHASE-DEPENDENT LINEAR CONDUCTANCE IN HIGHLY TRANSMISSIVE SUPERCONDUCTING QUANTUM POINT CONTACTS

A microscopic theory for transport properties of a superconducting qua ntum point contact is presented. According to contact transmission and quasiparticle damping two different physical regimes can be identifie d. In the limit of small applied voltages and weak damping this theory provides an exact analytical expression for the current through the c ontact. The phase-dependent linear conductance thus obtained exhibits an unusual behavior at low temperatures which strongly differs from th e standard cos phi-like form. It is argued that a realistic highly tra nsmissive contact will be accurately described by our analytical expre ssion.