  We study and characterize a new dynamical regime of underdamped particles in
a tilted washboard potential. We find that for small friction in a finite range
of forces the particles move essentially nondispersively, that is, coherently,
over long intervals of time. The associated distribution of the particle
positions moves at an essentially constant velocity and is far from
Gaussian-like. This new regime is complementary to, and entirely different
from, well-known nonlinear response and large dispersion regimes observed for
other values of the external force.
