We present a detailed analysis of the incoherent dynamic structure factor of a series of n-alkanes and high molar mass polyethylene (PE). C30H62 data indicate that two molecular processes are simultaneously active in our experimental temperature and time range but start to disentangle at low temperature. These are tentatively identified with torsion-vibrations and conformational relaxation. Our study fully supports a number of MD investigations and confirms the unique behavior of PE. For the first time, we demonstrate that the molecular processes detected by QENS for alkanes and PE occur on a similar time scale, and we therefore suggest that common molecular mechanisms are responsible for the decay of the intermediate scattering function. Interestingly, the molar mass dependence of the characteristic times and activation energies at the lowest Q investigated (1 angstrom(-1)) are comparable to those obtained from rheological measurements and follow a simple trend which can be accounted for by free volume theory. This surprisingly simple outcome suggests that local dynamics (e.g., conformational transitions) are intimately linked to the long-range motion. The surrounding medium has a global effect on the microscopic motion that can be modeled by the friction coefficient.