  The phenomenology of massive neutrinos -- flavour mixing in the lepton sector
causing oscillations between different neutrino-types along their propagation
over macroscopic distances in vacuum -- aims at relating observable quantities
(oscillation frequency or, equivalently, oscillation length) to the neutrino
properties: mixing angles $\theta_{ij}$ and mass-squared differences $\Delta
m_{ij}^2$.
  Calculation of the probabilities for a given neutrino-type either to survive
or to mutate into another type, as functions of momentum $p$ and travelling
distance $L$, are properly based on wave-packet models of varying complexity.
Approximations neglecting subtle effects like decoherence result in the
standard oscillation formulae with terms proportional to $\sin^2(\Delta
m_{ij}^2 L / 4 p)$.
  The same result may also be derived by a simple plane-wave model as shown in
most textbooks. However, those approaches rely on unphysical a-priory
assumptions: either "equal energy" or "equal velocity" or "equal momentum" in
the phases of different mass eigenstates -- which are refuted elsewhere. In
addition, some assume tacitly that interference occurs at time $t = L$.
  This study re-examines the plane-wave model. No unphysical assumption is
necessary for deriving the standard formulae: a heuristic approach relies only
on carefully defining interference at time $t = L / \beta$, and is justified by
coherence arguments based in a qualitative way on wave-packets.
