  Let $K$ be a finite extension of $\mathbb{Q}_p$, and choose a uniformizer
$\pi\in K$, and put $K_\infty:=K(\sqrt[p^\infty]{\pi})$. We introduce a new
technique using restriction to $\Gal(\ol K/K_\infty)$ to study flat deformation
rings. We show the existence of deformation rings for $\Gal(\ol
K/K_\infty)$-representations ``of height $\leqslant h$'' for any positive
integer $h$, and we use them to give a variant of Kisin's proof of connected
component analysis of a certain flat deformation rings, which was used to prove
Kisin's modularity lifting theorem for potentially Barsotti-Tate
representations. Our proof does not use the classification of finite flat group
schemes, so it avoids Zink's theory of windows and displays when $p=2$.
  This $\Gal(\ol K/K_\infty)$-deformation theory has a good analogue in
positive characteristics analogue of crystalline representations in the sense
of Genestier-Lafforgue. In particular, we obtain a positive characteristic
analogue of crystalline deformation rings, and can analyze their local
structure.
