SUBGAP STRUCTURE AS FUNCTION OF THE BARRIER IN ATOM-SIZE SUPERCONDUCTING TUNNEL-JUNCTIONS

Using a mechanically controllable breakjunction we study superconducti ng tunnel junctions of atom size. The vacuum barrier is adjusted by co ntrolling the distance between the electrodes to give barrier transpar encies ranging over 2 orders of magnitude. We observe a systematic var iation of the subgap current steps at eV = 2 Delta/n, for n = 2 and n = 3 in excellent agreement with theories of multiparticle tunneling an d multiple Andreev reflection.