The major contribution to bandspreading in SEC is the diffusion of the analyte, proteins or polymers, in and out of the pores. It is MUCH higher than the contribution from the A-term. The reason for this is two-fold. For one thing, large molecules diffuse slowly. In addition, once the analyte molecule starts to have a size that is within a factor of 10 of the pore size, diffusion slows down significantly, because the molecule is bumping agains the wall more often. Overall, there is a signficnat C term in SEC. It is also correct that this goes away for completely excluded peaks.