We identify the time $T$ between Andreev reflections as a classical adiabatic invariant in a ballistic chaotic cavity (Lyapunov exponent $\lambda$), coupled to a superconductor by an $N$-mode constriction. Quantization of the adiabatically invariant torus in phase space gives a discrete set of periods $T_n$, which in turn generate a ladder of excited states ${ϵ}_{nm}=(m+1/2)\pi \hslash /T_n$. The largest quantized period is the Ehrenfest time $T_0={\lambda }^{-1}\ln N$. Projection of the invariant torus onto the coordinate plane shows that the wave functions inside the cavity are squeezed to a transverse dimension $W/\sqrt{N}$, much below the width $W$ of the constriction.

• Received 12 August 2002

DOI:https://doi.org/10.1103/PhysRevLett.90.116801