Scanning tunneling potentiometry was successfully applied to evaluate thin InGaAs resistors with 30 nm spatial resolution. Measured potential distributions show large potential drops at the edge of and near the contacts. The observed potential drops at the edge of the contacts are attributed to Schottky barrier formation at the metal/InGaAs interface due to the low impurity concentration of InGaAs (n=1X10(18) cm-3). The potential drops observed near the contacts reflect the nonlinear carrier transport properties in InGaAs. They are ascribed to the carrier accumulation and/or depletion due to the carrier velocity dependence on the electric field. These potential drops well explain the measured current-saturation feature at low electric field.