  Coherent motion of cold atoms in a standing-wave field is interpreted as a
propagation in two optical potentials. It is shown that the wave-packet
dynamics can be either regular or chaotic with transitions between these
potentials after passing field nodes. Manifestations of de Broglie-wave chaos
are found in the behavior of the momentum and position probabilities and the
Wigner function. The probability of those transitions depends on the ratio of
the squared detuning to the Doppler shift and is large in that range of the
parameters where the classical motion is shown to be chaotic.
