Superconducting proximity effect and universal conductance fluctuations

We examine universal conductance fluctuations (UCF's) in mesoscopic normal-superconducting-normal (N-S-N) structures using a numerical solution of the Bogoliubov-de Gennes equation. We discuss two cases depending on the presence ("open" structure) or absence ("closed" structure) of quasiparticle transmission. In contrast to N-S structures, where the onset of superconductivity increases fluctuations, we find that UCF's are suppressed by superconductivity for N-S-N structures. We demonstrate that the fluctuations in ''open'' and "closed" structures exhibit distinct responses to an applied magnetic field and to an imposed phase variation of the superconducting order parameter.