The magnetoresistance of p‐Insb single crystal specimens with real surfaces is measured as a function of thickness, magnetic induction, temperature, and surface potential. Field effect and Hall coefficient are employed to determine the barrier height and the carrier concentrations during the investigations. The discrimination of the resistance change into ambipolar, scattering, inhomogeneity, and geometry effect is experimentally investigated by variation of sample thickness at different barrier height. The data are thoroughly discussed by the two‐conductor three‐band model taking into account the separation into electrons, light, and heavy holes.