  The average transverse momentum transfer per unit path length to a fast
parton scattering elastically in a perturbative quark-gluon plasma is related
to the radiative energy loss of the parton. We first calculate the momentum
transfer coefficient $\hat q$ in terms of a classical Langevin problem and then
define it quantum-mechanically through scattering matrix element. After
treating the well known case of a quark-gluon plasma in equilibrium we consider
an off-equilibrium unstable plasma. As a specific example, we treat the
two-stream plasma with unstable modes of longitudinal chromoelectric field. In
the presence of the instabilities, $\hat q$ is shown to exponentially grow in
time.
