Magnetic near fields as a probe of charge transport in spatially   dispersive conductors
  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. 
Harald R. Haakh, Carsten Henkel
cond-mat.mes-hall quant-ph