  The dynamics of a time-dependent stadium-like billiard are studied by a four
dimensional nonlinear mapping. We have shown that even without any dissipation,
the particle experiences a decrease on its velocity. Such condition is related
with a critical resonance velocity, where if the initial velocity has a higher
value than the resonant one, we can observe Fermi acceleration, however, if the
initial velocity has a initial value smaller than the critical one, the
particle is temporarily trapped surrounding the stability islands, in a
stickiness regime. We believe that this sticky orbits can act as deceleration
mechanism for Fermi Acceleration.
