  The recent conjecture of a topologically-protected surface state in SmB$_6$
and the verification of robust surface conduction below 4 K have prompted a
large effort to understand the surface states. Conventional Hall transport
measurements allow current to flow on all surfaces of a topological insulator,
so such measurements are influenced by contributions from multiple surfaces of
varying transport character. Instead, we study magnetotransport of SmB$_6$
using a Corbino geometry, which can directly measure the conductivity of a
single, independent surface. Both (011) and (001) crystal surfaces show a
strong negative magnetoresistance at all magnetic field angles measured. The
(011) surface has a carrier mobility of $122\text{
cm}^2/\text{V}\cdot\text{sec}$ with a carrier density of $2.5\times10^{13}
\text{ cm}^{-2}$, which are significantly smaller than indicated by Hall
transport studies. This mobility value can explain a failure so far to observe
Shubnikov-de Haas oscillations. Analysis of the angle-dependence of
conductivity on the (011) surface suggests a combination of a field-dependent
enhancement of the carrier density and a suppression of Kondo scattering from
native oxide layer magnetic moments as the likely origin of the negative
magnetoresistance. Our results also reveal a hysteretic behavior whose
magnitude depends on the magnetic field sweep rate and temperature. Although
this feature becomes smaller when the field sweep is slower, does not disappear
or saturate during our slowest sweep-rate measurements, which is much slower
than a typical magnetotransport trace. These observations cannot be explained
by quantum interference corrections such as weak anti-localization, but are
more likely due to an extrinsic magnetic effect such as the magnetocaloric
effect or glassy ordering.
