  We report photocarrier time-of-flight measurements in diode structures made
of highly porous crystalline silicon. The corresponding electron and hole drift
mobilities are very small compared to homogeneous crystalline silicon. They
show two additional signatures of semiclassical transport that are directly
limited by a porous geometry: the mobilities are dispersive (i.e. dependent on
time), and have very little temperature-dependence. A pure, geometry-limited
transport is surprising in the context of nanometer-scale models for electronic
states such as quantum percolation or nanocrystal aggregation. In such models,
confinement-induced energy shifts and localization are the dominant effects.
