The divergent at ω = 0 quantum correction to conductivity δ σ₂ (ω) of the leading order in (k_F l)^{- 1} has been calculated neglecting Cooperon-type contributions suppressed by moderate or strong magnetic field. In the so-called diffusion approximation this quantity is equal to zero up to the second order in (k_F l)^{- 1}. More subtle treatment of the problem shows that δ σ₂ (ω) is non-zero due to ballistic contributions neglected previously. Knowledge of δ σ₂ (ω) allows to estimate value of the so-called unitary localization length as ξ_u ≈ l exp (1.6 g²) where Drude conductivity is given by σ₀ = ge² / h. This estimation underpins the statement of the linear growth of σ_xx peaks with Landau level number n in the integer quantum Hall effect regime [1] (Greshnov and Zegrya, 2008; Greshnov et al., 2008) at least for n ≤ 2 and calls Pruisken-Khmelnitskii hypothesis of universality [2] (Levine et al., 1983; Khmelnitskii, 1983) in question.