I spoke to Jim Winjum today at Kenetrek boots reference this issue and he provided the following information to answer your question.
Heel slippage can be an issue with any stiff and supportive mountain boots especially when they are not broken in properly. Every persons heel and foot shape is complicated and completely different much like a finger print is. So it is important to find a boot that fits your foot the very best. The Kenetrek Mountain Boots have always been a very good fitting and performing boot occupying a “sweet” spot with stiff support good enough for most mountain hunting and flexible comfort that most of our customers love. When we first started out in 2006, the boot we offered had a fit rate of approximately 85% meaning that about 15% of our customers could not wear the boots with the same level of comfort as the rest. That “fit rate” was still the highest in the industry but I was not satisfied so Kenetrek set out to improve the fit and redesigned our own lasts. This project was enormously expensive but the results were extraordinary! By making adjustments to the heel pocket of the boots, we were able to increase the fit rate of our customers to an incredible 98%! We also added a premium molded footbed as a standard with our mountain boots making the current Kenetrek Mountain Boots the most comfortable mountain boots on the market. Even with this incredible fitting boot, it is still essential that our customers break the boots in properly before a big hunt. Any time a customer develops huge ugly blisters on their hunt, it tells me they did not properly prepare and wear the boots enough to make certain they fit their feet correctly. A back country sheep hunt is no place to find out your boots don’t fit! This is a matter of personal responsibility and it is like showing up on a hunt without sighting in your rifle then blaming the gun when you miss. Here is an article I wrote on selecting and fitting your mountain boots.
http://www.kenetrek.com/fitting-tips-mountainboots.asp
I hope some of this information helps.
Jim Winjum,
Kenetrek Boots