We have measured the low-field magnetoresistances of a series of Sn-doped indium oxide thick films in the temperature T range of . The electron dephasing rate as a function of T for each film was extracted by comparing the magnetoresistance data with the three-dimensional weak-localization theoretical predictions. We found that the extracted varies linearly with . Furthermore, at a given T, varies linearly with , where kF is the Fermi wave number, and l is the electron elastic mean free path. These features are well explained in terms of the small-energy-transfer electron-electron scattering time in three-dimensional disordered conductors. This electron dephasing mechanism dominates over the electron-phonon scattering process because the carrier concentrations in our films are ∼ 3 orders of magnitude lower than those in typical metals, which resulted in a greatly suppressed electron-phonon relaxation rate. Our result is the first quantitative demonstration of this unique three-dimensional electron-electron scattering time in experiments.