Sample-Specific Conductance Fluctuations Modulated by the Superconducting Phase

We have studied sample-specific conductance fluctuations tuned by the phase difference between superconducting boundaries, using a two-dimensional electron gas as the phase-coherent normal conductor. In low magnetic fields, $h/2e$ periodic conductance oscillations due to phase-conjugated Andreev reflections are observed with an amplitude $\delta G\simeq {0.10e}^2/h$ at $T=50\mathrm{mK}$. These oscillations are suppressed by a flux of approximately $h/e$ through the interference region. For larger magnetic fields, sample-specific $h/2e$ conductance oscillations are found with $\delta G\simeq {0.005e}^2/h$. No indication is found for an intrinsic $h/4e$ periodic superconductor-modified weak localization contribution.