  Theoretical results for the extension of a polymer confined to a channel are
usually derived in the limit of infinite contour length. But experimental
studies and simulations of DNA molecules confined to nanochannels are not
necessarily in this asymptotic limit. We calculate the statistics of the span
and the end-to-end distance of a semiflexible polymer of finite length in the
extended de Gennes regime, exploiting the fact that the problem can be mapped
to a one-dimensional weakly self-avoiding random walk. The results thus
obtained compare favourably with pruned-enriched Rosenbluth method (PERM)
simulations of a three-dimensional discrete wormlike chain model of DNA
confined in a nanochannel. We discuss the implications for experimental studies
of linear $\lambda$-DNA confined to nanochannels at the high ionic strengths
used in many experiments.
