Josephson current carried by Andreev levels in superconducting quantum point contacts

The dc Josephson effect in a superconducting quantum point contact, where supercurrent flows through a small number of channels, is reviewd. The central role of Andreev levels is emphasized which carry the whole supercurrent in short symmetric Josephson junctions including tunnel junctions. A simple intuitive view of the dc Josephson effect in a quantum point contact is given in terms of multiple Andreev reflections. The quantization of the critical current in superconducting quantum point contacts is briefly discussed.