  We describe a phenomenological model for the conductance feature near $0.7
\times 2e^2/h$ that occurs in quantum point contacts. We focus on the
transconductance at finite source-drain bias and contrast our model with the
results expected from a single-particle picture. Good agreement is seen in
comparing the model with experimental data, taken on ultra-low-disorder GaAs
induced electron systems. Although simple, our phenomenology suggests important
boundary conditions for an underlying microscopic theory.
