Nonreciprocity of the wave-packet scattering delay in ballistic two-terminal devices

In the linear regime, transport properties of ballistic two-terminal devices are generally considered to be independent of the direction of the current. This two-terminal reciprocity applies to both the electron transmission and reflection probabilities. However, it does not apply to the scattering delays of wave packets. Indeed, four different time delays describe the transmission and reflection processes of wave packets arriving from the two terminals, respectively. Unlike the probabilities, these delays are direction dependent if the channel exchange symmetry of the scattering matrix is broken.