  We study the behavior of a moving wall in contact with a particle gas and
subjected to an external force. We compare the fluctuations of the system
observed in the microcanonical and canonical ensembles, at varying the number
of particles. Static and dynamic correlations signal significant differences
between the two ensembles. Furthermore, velocity-velocity correlations of the
moving wall present a complex two-time relaxation which cannot be reproduced by
a standard Langevin-like description. Quite remarkably, increasing the number
of gas particles in an elongated geometry, we find a typical timescale, related
to the interaction between the partitioning wall and the particles, which grows
macroscopically.
