  Elastic $^{6}$He+$^4$He data measured at $E_{\rm c.m.}=11.6,$ 15.9, and 60.3
MeV have been analyzed within the coupled reaction channels (CRC) formalism,
with the elastic-scattering and two-neutron ($2n$) transfer amplitudes
coherently included. Contributions from the direct (one-step) and sequential
(two-step) $2n$-transfers were treated explicitly based on a realistic
assumption for the $2n$-transfer form factor. The oscillatory pattern observed
in $^4$He($^6$He,$^6$He)$^4$He angular distribution at low energies was found
to be due to an interference between the elastic scattering and $2n$-transfer
amplitudes. Our CRC analysis shows consistently that the direct $2n$-transfer
strongly dominates over the sequential transfer and thus confirms the dominance
of 2$n-^4$He configuration over the $n-^5$He one in the $^6$He wave function.
This result suggests a strong clusterization of the two valence neutrons and
allows, therefore, a reliable estimate for the \emph{di-neutron} spectroscopic
amplitude.
