  We calculate magnetic field fluctuations above a conductor with a nonlocal
response (spatial dispersion) and consider a large range of distances. The
cross-over from ballistic to diffusive charge transport leads to reduced noise
spectrum at distances below the electronic mean free path, as compared to a
local description. We also find that the mean free path provides a lower limit
to the correlation (coherence) length of the near field fluctuations. The
short-distance behavior is common to a wide range of materials, covering also
semiconductors and superconductors. Our discussion is aimed at atom chip
experiments where spin-flip transitions give access to material properties with
mesoscopic spatial resolution. The results also hint at fundamental limits to
the coherent operation of miniaturized atom traps and matter wave
interferometers.
