Generally nobody has had much luck with wall warts rated under 1A, my own is built from an old Linksys network router PSU (1.5A) wired to a USB socket (and I just use my pass
through). I use 801's, so I'm near the edge of its rated capacity. My car 12V->USB adapter is rated at 1A and is probably dropping voltage down to 4V or so under load (the LED nearly goes out), although I haven't measured it (keep meaning to make an extension that would make measuring loaded voltage and current easy, but never get around to it).
With the starting point voltage being almost minimum to begin with, and such an extreme under-rating for the load, it's unlikely that one will work.
--Dave