  The local phase-invariance of the momentum-space Schr\"odinger equation for
free-particle has been used to construct quantum kinematics that describes a
motion of the particle in external U(1) background gauge field. The gauge
structure over the momentum space of the particle is interpreted in terms of
helicity and spin carried by the particle. As a by-product an effective
one-particle Schr\"odinger equation of motion for the helicity-carrying
particle in external potential field is derived. An effect of screening of the
external potential is predicted, that can affect quantization and splittings of
energy levels of the particle.
