The expansion of a Bose-Einstein condensate in an atomic waveguide is
analyzed. We study different regimes of expansion, and identify a transient
regime between one-dimensional and three-dimensional dynamics, in which the
properties of the condensate and its further expansion can be well explained by
reducing the transversal dynamics to a two-level system. The relevance of this
regime in current experiments is discussed.Comment: 4 pages, 3 figs, Accepted for publication in Phys. Rev.