Summary form only given. Optical intersubband (IS) transitions in low-dimensional semiconductors are strongly affected by carrier interactions on ultrafast timescales. Coherent IS polarizations provide particularly sensitive probes of these fundamental interactions and, moreover, influence the characteristics of novel devices such as the quantum cascade laser. Most studies have investigated the energy position and inhomogeneous broadening of IS transitions? In contrast, homogeneous broadening is much less understood. Nonlinear optical techniques provide the key to characterize these irreversible dephasing processes. Here, we carry out time-integrated and-for the first time-time-resolved four-wave-mixing (FWM) in the midinfrared to investigate IS dephasing in n-type GaAs quantum wells with negligible disorder, for different electron densities and excitation intensities.