  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, \emph{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.
