  We study the diffusive dynamics of a hard-sphere fluid confined between
parallel smooth hard walls. The position-dependent diffusion coefficient normal
to the walls is larger in regions of high local packing density. High density
regions also have the largest available volume, consistent with the fast local
diffusivity. Indeed, local and global diffusivities as a function of the Widom
insertion probability approximately collapse onto a master curve. Parallel and
average normal diffusivities are strongly coupled at high densities and deviate
from bulk fluid behavior.
