JOSEPHSON CURRENT THROUGH A SUPERCONDUCTING QUANTUM POINT CONTACT SHORTER THAN THE COHERENCE LENGTH

It is shown theoretically that the critical current of a smooth and impurity-free superconducting constriction increases stepwise as a function of its width. The step height at zero temperature is e-DELTA-0/hBAR dependent on the energy gap DELTA-0 of the bulk superconductor but not on the junction parameters. This is the analog of the quantized conductance of a point contact in the normal state. It is essential that the constriction is short compared to the superconducting coherence length.