  Restrictions to diffusion result in the dispersion of the bulk diffusion
coefficient. We derive the exact universal high-frequency behavior of the
diffusion coefficient in terms of the surface-to-volume ratio of the
restrictions. This frequency dependence can be applied to quantify structure of
complex samples with NMR using oscillating field gradients and static-gradient
CPMG. We also demonstrate the inter-relations between different equivalent
diffusion metrics, and describe how to calculate the effect of restrictions for
arbitrary gradient waveforms.
