  We use quantum scattering theory to study a fixed quantum system Y subject to
collisions with massive particles X described by wave-packets. We derive the
scattering map for system Y and show that the induced evolution crucially
depends on the width of the incident wave-packets compared to the level spacing
in Y . If Y is non-degenerate, sequential collisions with narrow wave-packets
cause Y to decohere. Moreover, an ensemble of narrow packets produced by
thermal effusion causes Y to thermalize. On the other hand, broad wave-packets
can act as a source of coherences for Y , even in the case of an ensemble of
incident wave-packets given by the effusion distribution, preventing
thermalization. We illustrate our findings on several simple examples and
discuss the consequences of our results in realistic experimental situations.
