  We investigate the photocurrent properties of the topological insulator
(Bi$_{0.5}$Sb$_{0.5}$)$_2$Te$_3$ on SrTiO$_3$-substrates. We find reproducible,
submicron photocurrent patterns generated by long-range chemical potential
fluctuations, occurring predominantly at the topological insulator/substrate
interface. We fabricate nano-plowed constrictions which comprise single
potential fluctuations. Hereby, we can quantify the magnitude of the disorder
potential to be in the meV range. The results further suggest a dominating
photo-thermoelectric current generated in the surface states in such nanoscale
constrictions.
