In order to study the mechanisms limiting the topological chain confinement in polymer melts, we have performed neutron spin echo investigations of the single chain dynamic structure factor from polyethylene melts over a large range of chain lengths. While at high molecular weight the reptation model is corroborated, a systematic loosening of the confinement with decreasing chain length is found. The dynamic structure factors are quantitatively described by the effect of contour length fluctuations on the confining tube establishing this mechanism on a molecular level in space and time.