Sum-over-histories origin of the composition laws of relativistic   quantum mechanics and quantum cosmology
  The scope of the paper has been broadened to include a more complete discussion of the following topics: The derivation of composition laws in quantum cosmology. The connection between the existence of a composition law in the sum over histories approach to relativistic quantum mechanics and quantum cosmology, and the existence of a canonical formulation. 
Jonathan Halliwell and Miguel Ortiz