  In the traditional Kaluza-Klein theory, the cylinder condition and the
constancy of the extra-dimensional radius (scalar field) imply that timelike
geodesics on the 5-dimensional bundle project to solutions of the Lorentz force
equation on spacetime. This property is lost for non constant scalar fields, in
fact there appear new terms that have been interpreted mainly as new forces or
as due to a variable inertial mass and/or charge. Here we prove that the
additional terms can be removed if we assume that charged particles are coupled
with the same spacetime conformal structure of neutral particles but through a
different conformal factor. As a consequence, in Kaluza-Klein theory the proper
time of the charged particle might depend on the charge-to-mass ratio and the
scalar field. Then we show that the compatibility between the equation of the
projected geodesic and the classical limit of the Klein-Gordon equation fixes
unambiguously the conformal factor of the coupling metric solving the
`conformal ambiguity problem' of Kaluza-Klein theories. We confirm this result
by explicitly constructing the projection of the Klein-Gordon equation and by
showing that each Fourier mode, even for a variable scalar field, satisfies the
Klein-Gordon equation on the base.
