  Resistance in superconductors arises from the motion of vortices driven by
flowing supercurrents or external electromagnetic fields and may be strongly
affected by thermal or quantum fluctuations. The common expectation borne out
in previous experiments is that as the temperature is lowered, vortex motion is
suppressed, leading to a decreased resistance. A new generation of materials
provides access to the previously inaccessible regime of clean-limit
superconductivity in atomically thin superconducting layers. We show
experimentally that for few-layer 2H-NbSe$_2$ the resistance below the
superconducting transition temperature may be non-monotonic, passing through a
minimum and then increasing again as temperature is decreased further. The
effects exists over a wide range of current and magnetic fields, but is most
pronounced in monolayer devices at intermediate currents. Analytical and
numerical calculations confirm that the findings can be understood in a
two-fluid vortex model, in which a fraction of vortices flow in channels while
the rest are pinned but thermally fluctuating in position. We show
theoretically that the pinned, fluctuating vortices effectively control the
mobility of the free vortices. The findings provide a new perspective on
fundamental questions of vortex mobility and dissipation in superconductors.
