THERMAL NOISE IN SUPERCONDUCTING QUANTUM POINT CONTACTS

We present a theory for the frequency-dependent current fluctuations i n superconducting quantum point contacts (SQPC) within the dc transpor t regime. This theory is valid for any barrier transparency between th e tunnel and ballistic limits, yielding an analytical expression for t he fluctuations spectrum in the subgap region. It is shown that the le vel of noise in a quasiballistic SQPC may have a huge increase in comp arison with the case of a normal contact carrying the same average cur rent. The effect of this high level of noise on the actual observabili ty of the current-phase relation for a ballistic point contact is disc ussed in connection with recent experimental measurements.