  The peculiar nature of electron scattering in graphene is among many exciting
theoretical predictions for the physical properties of this material. To
investigate electron scattering properties in a graphene plane, we have created
a gate-tunable potential barrier within a single-layer graphene sheet. We
report measurements of electrical transport across this structure as the
tunable barrier potential is swept through a range of heights. When the barrier
is sufficiently strong to form a bipolar junctions (npn or pnp) within the
graphene sheet, the resistance across the barrier sharply increases. We compare
these results to predictions for both diffusive and ballistic transport, as the
barrier rises on a length scale comparable to the mean free path. Finally, we
show how a magnetic field modifies transport across the barrier.
