  A simple kinematical argument suggests that the classical approximation may
be inadequate to describe the evolution of a system with an anisotropic
particle distribution. In order to verify this quantitatively, we study the
Boltzmann equation for a longitudinally expanding system of scalar particles
interacting with a $\phi^4$ coupling, that mimics the kinematics of a heavy ion
collision at very high energy. We consider only elastic $2\to 2$ scatterings,
and we allow the formation of a Bose-Einstein condensate in overpopulated
situations by solving the coupled equations for the particle distribution and
the particle density in the zero mode. For generic CGC-like initial conditions
with a large occupation number and a moderate coupling, the solutions of the
full Boltzmann equation do not follow a classical attractor behavior.
