The energy width Delta omega of the quasielastic coherent dynamic structure factor S(Q, omega) for a simple liquid exhibits the oscillating dependence on wavenumber Q with the sharp minimum at Q(max) corresponding to the maximum of the structure factor S(Q). The only known expression for Delta omega(Q) was derived for a dense hard-sphere (HS) fluid (Cohen et al., Phys. Rev. Lett. 1987, 59, 2872). Though this expression has been frequently used for the analysis of the experimental data obtained for liquid metals, until now, it has never been tested against a true HS fluid. A test performed by means of HS molecular dynamic simulations reveals a considerable discrepancy between the simulations results and the examined model. The main output of the analysis is the finding that the Delta omega(HS)(Q) behavior is defined in terms of the average cage size, < L-c >, rather than of the sigma(HS) diameter, ohs. The simulated Delta omega(HS)(Q) has been compared with the results for the soft-spherical potential. The microscopic dynamics of the soft-sphere fluid shows significant difference in comparison to the HS system. Nevertheless, the diffusive mobility of soft spheres can be characterized within the HS approximation using an effective diameter, sigma(eff), and this parameter can be found from Delta omega(Q) at Q approximate to Q(max) A similar result has been obtained for the neutron scattering data measured for liquid Rb.