  We consider a novel quantum field-theoretical approach to the description of
processes passing at finite space-time intervals based on the Feynman diagram
technique in the coordinate representation. The most known processes of this
type are neutrino and neutral kaon oscillations. The experimental setting of
these processes requires one to adjust the rules of passing to the momentum
representation in the Feynman diagram technique in accordance with it, which
leads to a modification of the Feynman propagator in the momentum
representation. The approach does not make use of wave packets, both initial
and final particle states are described by plane waves, which simplifies the
calculations considerably. We consider neutrino oscillation processes, where
the neutrinos are produced in three-particle weak decays of nuclei and detected
in the charged-current interaction with nuclei or in the charged- and
neutral-current interactions with electrons. Particular examples are considered
and it is shown that the momentum spread of the produced neutrinos and the
energy dependence of the differential cross section of the detection process
result in the suppression of neutrino oscillation, which is characterized by a
coherence length specific for a pair of production and detection processes.
This coherence length turns out to be much less than the coherence length in
the standard quantum-mechanical approach defined by the quantum uncertainty of
neutrino momentum.
