  Using the functional-integral method, we investigate the effect of a
two-level systems thermal reservoir on the single particle dynamics. We find
that at low temperatures, within the sub-ohmic regime, the particle becomes
{}``dynamically'' localized at long times due to an effective potential
generated by the particle-reservoir interaction. This behavior is different
from the one obtained for the usual bath of harmonic oscillators and is
fundamentally related with the non-Markovian character of the dissipative
process.
