  Topological insulators are characterized by an inverted band structure in the
bulk and metallic surface states on the surface. In LaBi, a semimetal with a
band inversion equivalent to a topological insulator, we observe surface state
like behavior in the magnetoresistance. The electrons responsible for this
pseudo two dimensional transport, however, originate from the bulk states
rather topological surface states, which is witnessed by the angle dependent
quantum oscillations of the magnetoresistance and ab initio calculations. As a
consequence, the magnetoresistance exhibits strong anisotropy with large
amplitude (~ 10^5 %).
