Quantum-coherent phenomena in mesoscopic proximity structures

We have investigated the magnetic response of a Ag cylinder in proximity with Nb at 300 mu K less than or equal to T less than or equal to 9 K and in the very clean limit. Below a temperature at which the proximity decay length xi(N) = <(h)over bar v(F)/2 pi k(B)T> becomes of the order of the perimeter L (xi(N) similar or equal to 0.2L), we observe a reentrance of the induced diamagnetic susceptibility chi(d). The reentrant susceptibility chi(tau) follows chi(tau) = A exp(-T/T*) with T* = <(h)over bar v(F)/2 pi k(B)L>. At T = T*, where xi(N) = L, the logarithmic slope of chi(pi) becomes steeper by a factor of 2 so that for T < T*, anew, stronger mesoscopic regime is reached with chi(tau) = B exp(-2T/T*).