Transmission resonances due to an impurity in a quantum point contact or narrow channel have been studied theoretically in Refs.241,407, and408. In an experiment it may be difficult to distinguish these resonances from those associated with reflection at the entrance and exit of the quantum point contact (discussed in Section III.B). A conductance peak associated with resonant tunneling through an impurity state in a quantum point contact was reported by McEuen et al.409 The experimental results