Einstein and the Kaluza-Klein particle
  In his search for a unified field theory that could undercut quantum mechanics, Einstein considered five dimensional classical Kaluza-Klein theory. He studied this theory most intensively during the years 1938-1943. One of his primary objectives was finding a non-singular particle solution. In the full theory this search got frustrated and in the x^5-independent theory Einstein, together with Pauli, argued it would be impossible to find these structures. 
Jeroen van Dongen