What is the entanglement length in a polymer melt?

We present the results of molecular dynamics simulations of very long model polymer chains analyzed by various experimentally relevant techniques. The segment motion of the chains is found to be in very good agreement with the reptation model. For comparison with experimental results we also calculated the plateau modulus $G_N^0$ via stress-strain simulations. The predictions of the entanglement length Ne from $G_N^0$ and from the mean-square dispacement of the chain segments disagree by a factor of about 2.3(2), indicating an error in the prefactor in the standard formula for $G_N^0$ which is important for the analysis of many simulation results as well as the interpretation of experimental neutron spin echo (NSE) and NMR measurements. We explicitly show how recent NSE measurements are affected by this and some finite-size effects in the interpretation of the dynamic structure factor.