  We study the transport properties of the Kondo insulator SmB$_6$ with a
specialized configuration designed to distinguish bulk-dominated conduction
from surface-dominated conduction. We find that as the material is cooled below
4 K, it exhibits a crossover from bulk to surface conduction with a fully
insulating bulk. We take the robustness and magnitude of the surface
conductivity, as is manifest in the literature of SmB$_6$, to be strong
evidence for the topological insulator metallic surface states recently
predicted for this material.
