Low temperature transport measurements have been performed on nanofabricated Au wires a few μm long contacting two Nb electrodes whose superconducting phase difference $\Delta \theta$ was externally controlled. $\Delta \theta$ was generated by passing a known supercurrent through a series array of $28 \mathrm{N}\mathrm{b}-\mathrm{A}\mathrm{l}{\mathrm{O}}_x-\mathrm{N}\mathrm{b}$ Josephson junctions. The conductance of the Au wire displays a component in $\Delta \theta$ with period $2\pi$. These conductance oscillations arise from the change in scattering boundary conditions of quasiparticles in the Au wire at the Au-Nb interfaces due to Andreev reflection.

• Received 29 March 1994

DOI:https://doi.org/10.1103/PhysRevLett.73.1416