  Using the axially-symmetric time-dependent Gross-Pitaevskii equation we study
the phase coherence in a repulsive Bose-Einstein condensate (BEC) trapped by a
harmonic and an one-dimensional optical lattice potential to describe the
experiment by Cataliotti {\it et al.} on atomic Josephson oscillation [Science
{\bf 293}, 843 (2001)]. The phase coherence is maintained after the BEC is set
into oscillation by a small displacement of the magnetic trap along the optical
lattice. The phase coherence in the presence of oscillating neutral current
across an array of Josephson junctions manifests in an interference pattern
formed upon free expansion of the BEC. The numerical response of the system to
a large displacement of the magnetic trap is a classical transition from a
coherent superfluid to an insulator regime and a subsequent destruction of the
interference pattern in agreement with the more recent experiment by Cataliotti
{\it et al.} [e-print cond-mat/0207139].
