From Sanderson's blog, written when the book was released: "The sequence (it is more than one scene) that I am referring to most of the time when I talk about this encompasses the entire epilogue of A Memory of Light. Once you get there, you can know you're reading Robert Jordan's words, though of course there are other scenes scattered through the book that he worked on too."
The entire chapter was written by Jordan, though I believe Sanderson/Harriet added a few small tweaks.
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