Andreev reflection and quantum transport in an S-N-S junction

The superconducting and normal transport in a semiconductor-coupled Josephson junction with a submicron split gate electrode has been studied. The maximum supercurrent I-c and the conductance show step-wise changes as a function of gate voltage V-g. This is evidence that I-c is quantized in the superconducting quantum point contact. The current-voltage characteristics also change from current-deficit to excess-current as the absolute value of V-g increases. This change is explained by Andreev reflection, which leads to the reflected holes being focused on the quantum point contact. To study the change, the dependence of the gate voltage and the magnetic field of the differential resistance-voltage characteristics are measured and discussed.