  We identify the different contributions to quantum interference in a
mesoscopic metallic loop in contact with two superconducting electrodes. At low
temperature, a flux-modulated Josephson coupling is observed with strong
damping over the thermal length L_{T}. At higher temperature, the
magnetoresistance exhibits large h/2e-periodic oscillations with 1/T power law
decay. This flux-sensitive contribution arises from coherence of low-energy
quasiparticles states over the phase-breaking length L_{\phi}. Mesoscopic
fluctuations contribute as a small h/e oscillation, resolved only in the purely
normal state.
