  We take graphene as an example to demonstrate that the present widely adopted
expression is only the scattering component of a true 2D group delay in the
condensed matter context, in which the spatial Goos-H\"{a}nchen (GH) shift
along an interface contributes an intrinsic component. We relate the dwell time
to spin precession and derive a relation between the 2D group delay and dwell
time, whereby we for the first time reveal that, the group delay for 2D
ballistic electronic systems can be directly observed by measuring a
conductance difference in a weak-field spin precession experiment. This
physical observable not only implies the group delay being a relevant quantity
even in the condensed matter context, but also provides an experimental
evidence for the intrinsic effect of the GH shift. Finally, we revisit the 2D
Hartman effect, a central issue of the group delay, by analytically solving it
via the vested relation and calculating the proposed observable at the Dirac
point.
