  Ultracold confined one-dimensional atomic gases are predicted to support dark
soliton solutions arising from a nonlinear Schr\"{o}dinger equation of suitable
nonlinearity. In weakly-interacting (high density) gases, the nonlinearity is
cubic, whereas an approximate model for describing the behaviour of strongly -
interacting (low density) gases is one characterized by a quintic nonlinearity.
We use an approximate analytical expression for the form of the nonlinearity in
the intermediate regimes to show that, near the crossover between the two
different regimes, the soliton is predicted and numerically confirmed to
oscillate at a frequency of $\sqrt{2/3}\Omega$, where $\Omega$ is the harmonic
trap frequency.
