  Molecular dynamics simulations of sorbates of different sizes confined to the
interior of carbon nanotubes are reported. The mean squared displacement shows
gradual change from diffusive for small sorbates to superdiffusive for
intermediate sized-sorbates to ballistic for sizes comparable to the channel
diameter. We show that this crossover behaviour can be understood on the basis
of a gradual decrease of the x-y component of the force with the levitation
parameter. The analysis can also help to rationalize some recently published
results.
