  An electric field that builds in the direction against current, known as
negative nonlocal resistance, arises naturally in viscous flows and is thus
often taken as a telltale of this regime. Here we predict negative resistance
for the ballistic regime, wherein the ee collision mean free path is greater
than the length scale at which the system is being probed. Therefore, negative
resistance alone does not provide strong evidence for the occurrence of the
hydrodynamic regime; it must thus be demoted from the rank of a smoking gun to
that of a mere forerunner. Furthermore, we find that negative response is
log-enhanced in the ballistic regime by the physics related to the seminal
Dorfman-Cohen log divergence due to memory effects in the kinetics of dilute
gases. The ballistic regime therefore offers a unique setting for exploring
these interesting effects due to electron interactions.
