  We report the observation of the superconducting proximity effect in
nanoribbons of a candidate topological insulator (Bi2Se3) which is interfaced
with superconducting (tungsten) contacts. We observe a supercurrent and
multiple Andreev reflections for channel lengths that are much longer than the
inelastic and diffusive thermal lengths deduced from normal state transport.
This suggests that the proximity effect couples preferentially to a ballistic
surface transport channel, even in the presence of a coexisting diffusive bulk
channel. When a magnetic field is applied perpendicular to the plane of the
nanoribbon, we observe magnetoresistance oscillations that are periodic in
magnetic field. Quantitative comparison with a model of vortex blockade relates
the occurrence of these oscillations to the formation of Pearl vortices in the
region of proximity induced superconductivity.
