  Motivated by recent experiments [Claussen N R et al. 2003 Preprint
cond-mat/0302195] we investigate the magnetic-field dependence of the Josephson
frequency of coherent atom-molecule oscillations near a Feshbach resonance. Far
off resonance this frequency is determined by two-body physics. However, the
Josephson frequency is relatively large and a description purely in terms of
scattering lengths turns out to be inadequate. In particular, we have to
include the effective range parameter of the interatomic interaction in our
calculations. Close to resonance, the frequency deviates from the two-body
results due to various many-body shifts. Considering also the many-body
effects, we find perfect agreement with the experimental results over the
entire range of magnetic field.
