High-frequency radiation detection by metal-metal point contacts is investigated as a function of bias voltage at far-infrared frequencies. Detection occurs through rectification due to a nonlinearity of the current-voltage characteristic. The relative contribution to the rectification due to an electron phonon scattering mechanism occurring in the bulk material (]]spreading resistance) is compared with other mechanisms. The spreading resistance nonlinearity was identified by measurements at liquid helium temperatures. For low contact resistances (≲50 Ω) this nonlinearity was generally dominant, for higher resistances it may occur in addition to and independently of the other mechanisms.