I just found this out too. I sized down by a half from my TTS on the Barrie LWB and noticed some heel slippage. It doesn't feel as though a smaller size would correct the problem because my toes had the right amount of room. Any suggestions on how to remedy this wide heel problem? Wearing them sockless actually helped make my foot catch more at the heel, allowing for a less slippage. At least I know I didn't go too big by the sound of it from some of you guys.
Try a tongue pad or an insole. I am between sizes in a few of my shoes. For some, a tongue pad does the trick, for others, a thin insole fixes it. For example, I am between sizes in Tru-Balanace and a thin insole helped. For a pair of shoes by another manufacturer, a tongue pad fixed some heel slip.