  We have studied the properties of the non-classical behavior of atoms in a
double-slit interferometer. An indication of this behavior for metastable
helium was reported by Kurtsiefer, Pfau and Mlynek [Nature 386, 150 (1997)]
showing distinctive negative values of the Wigner function, which was
reconstructed from the measured diffraction data. Our approach to explain this
non-classical behavior is based on the de Broglie-Bohm-Vigier-Selleri
understanding of the wave-particle duality and compatible statistical
interpretation of the atomic wave function. It follows from the results that
the atomic motion is non-classical because it does not obey the laws of
classical mechanics. However, there is no evidence that this atomic behavior
violates the classical probability law of the addition of probabilities.
