Conduction channels of superconducting quantum point contacts

Atomic quantum point contacts accommodate a small number of conduction channels. Their number N and transmission coe$cients M1 n N can be determined by analyzing the subgap structure due to multiple Andreev re#ections in the current}voltage (I}<) characteristics in the superconducting state. With the help of mechanically controllable break-junctions we have produced Al contacts consisting of a small number of atoms. In the smallest stable contacts, usually three channels contribute to the transport. We show here that the channel ensemble M1 n N of few atom contacts remains unchanged up to temperatures and magnetic "elds approaching the critical temperature and the critical "eld, respectively, giving experimental evidence for the prediction that the conduction channels are the same in the normal and in the superconducting state. ( 2000 Elsevier Science B.V. All rights reserved.