  Neutrino flavor mixing is discussed in terms of two-component coupled
left-handed flavor fields. This is to take into account the fact that the weak
interaction couples only to left-handed fields. The flavor fields are written
through a rotation matrix, as a linear combination of left-handed free fields.
In order to obtain properly normalized wave functions directly from those free
fields, states of mixed helicity have to be considered. Neutrino flavor
oscillation amplitudes are also derived.
