  We consider electron-electron interaction effects in quantum point contacts
on the first quantization plateau, taking into account all scattering
processes. We compute the low-temperature linear and nonlinear conductance,
shot noise, and thermopower, by perturbation theory and a self-consistent
nonperturbative method. On the conductance plateau, the low-temperature
corrections are solely due to momentum-nonconserving processes that change the
relative number of left- and right-moving electrons. This leads to a
suppression of the conductance for increasing temperature or voltage. The size
of the suppression is estimated for a realistic saddle-point potential, and is
largest in the beginning of the conductance plateau. For large magnetic field,
interaction effects are strongly suppressed by the Pauli principle, and hence
the first spin-split conductance plateau has a much weaker interaction
correction. For the nonperturbative calculations, we use a self-consistent
nonequilibrium Green's function approach, which suggests that the conductance
saturates at elevated temperatures. These results are consistent with many
experimental observations related to the so-called 0.7 anomaly.
