We identify the time T between Andreev reflections as a classical adiabatic invariant in a ballistic chaotic cavity (Lyapunov exponent λ), coupled to a superconductor by an N -mode constriction. Quantization of the adiabatically invariant torus in phase space gives a discrete set of periods Tn, which in turn generate a ladder of excited states εnm = (m + 1/2)πh/Tn. The largest quantized period is the Ehrenfest time T0 = λ −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/ √ N , much below the width W of the constriction.PACS numbers: 05.45. Mt, 73.63.Kv, 74.50.+r, 74.80.Fp The notion that quantized energy levels may be associated with classical adiabatic invariants goes back to Ehrenfest and the birth of quantum mechanics [1]. It was successful in providing a semiclassical quantization scheme for special integrable dynamical systems, but failed to describe the generic nonintegrable case. Adiabatic invariants play an interesting but minor role in the quantization of chaotic systems [2,3].Since the existence of an adiabatic invariant is the exception rather than the rule, the emergence of a new one quite often teaches us something useful about the system. An example from condensed matter physics is the quantum Hall effect, in which the semiclassical theory is based on two adiabatic invariants: The flux through a cyclotron orbit and the flux enclosed by the orbit center as it slowly drifts along an equipotential [4]. The strong magnetic field suppresses chaotic dynamics in a smooth potential landscape, rendering the motion quasiintegrable.Some time ago it was realized that Andreev reflection has a similar effect on the chaotic motion in an electron billiard coupled to a superconductor [5]. An electron trajectory is retraced by the hole that is produced upon absorption of a Cooper pair by the superconductor. At the Fermi energy E F the dynamics of the hole is precisely the time reverse of the electron dynamics, so that the motion is strictly periodic. The period from electron to hole and back to electron is twice the time T between Andreev reflections. For finite excitation energy ε the electron (at energy E F + ε) and the hole (at energy E F − ε) follow slightly different trajectories, so the orbit does not quite close and drifts around in phase space. This drift has been studied in a variety of contexts [5,6,7,8,9], but not in connection with adiabatic invariants and the associated quantization conditions. It is the purpose of this paper to make that connection and point out a striking physical consequence: The wave functions of Andreev levels fill the cavity in a highly nonuniform "squeezed" way, which has no counterpart in normal state chaotic or regular billiards. In particular the squeezing is distinct from periodic orbit scarring [10] and entirely different from the random superposition of plane waves expected for a fully chaotic billiard [11].Adiabatic quantization breaks down near the excitation g...