  In solids and organic materials, environment-induced dephasing of particles
and long-lived excitations leads to the crossover in their transport properties
between quantum wave-like propagation and classical diffusive motion. In this
work, we demonstrate that dynamics of single carriers in this intermediate
crossover regime can exhibit distinct signatures such as the formation of
vortices and viscous flow, the phenomena typically considered as manifestations
of hydrodynamic transport. We explain this effect by modeling suppressed
quantum interference of carriers, and we show that the resulting dynamics
resembles the linearized Navier-Stokes equations. Dephasing-assisted viscosity
provides a potential alternative explanation of the results of recent
experiments exhibiting hydrodynamic behavior in solids, and suggests
experimental probes of how quantum carriers couple to their environment.
