The process of converting values to TOA Reflectance (L1C product) into TOA spectral radiance is described on the page:
http://s2tbx.telespazio-vega.de/sen2three/html/r2rusage.html?highlight=quantification
rToa = (float)(DN_L1C_band / QUANTIFICATION_VALUE);
L = (rToa * e0__SOLAR_IRRADIANCE_For_band * cos(Z__Sun_Angles_Grid_Zenith_Values)) / (PI * U__earth_sun_distance_correction_factor);
The desired values can be found in the xml file of the L1C product.
However, there is one weird thing that I encountered during processing.
It is associated with the presence data whose values exceed QUANTIFICATION_VALUE.
For example, data for 2016 QUANTIFICATION_VALUE in the scene with the snow coverage is 10,000!
But, most of the values in this scene exceeds QUANTIFICATION_VALUE. While my question is how to be in this case – the answer is no.