Conduction channels of superconducting quantum point contacts

Atomic quantum point contacts accommodate a small number of conduction chan nels. Their number N and transmission coefficients {T-n} can be determined by analyzing the subgap structure due to multiple Andreev reflections in th e current-voltage (I-V) characteristics in the superconducting state. With the help of mechanically controllable break-junctions we have produced Al c ontacts consisting of a small number of atoms. In the smallest stable conta cts? usually three channels contribute to the transport. We show here that the channel ensemble {T-n} of few atom contacts remains unchanged up to tem peratures and magnetic fields approaching the critical temperature and the critical held, respectively, giving experimental evidence for the predictio n that the conduction channels are the same in the normal and in the superc onducting state. (C) 2000 Elsevier Science B.V. All rights reserved.