  We treat the nonequilibrium motion of a single impurity atom in a
low-temperature single-species Fermi sea, interacting via a contact
interaction. In the nonequilibrium regime, the impurity does a superdiffusive
geometric random walk where the typical distance traveled grows with time as
$\sim t^{d/(d+1)}$ for the $d$-dimensional system with $d\geq 2$. For nonzero
temperature $T$, this crosses over to diffusive motion at long times with
diffusivity $D\sim T^{-(d-1)/2}$. These results apply also to a nonzero
concentration of impurity atoms as long as they remain dilute and
nondegenerate.
