When I listen to the words at learnersdictionary.com, I hear an unvoiced plosive in plenty and a voiced tap in particle. However, my ear is not particularly good these days, and I no longer have access to a spectogram.
Unfortunately, my LPD CD is not working, so I cannot check on the words Wells was transcribing. Interestingly, the Cambridge English Pronouncing Dictionary agrees with Wells' LPD. That's all I can really say on the subject, I am afraid. As I am not a speaker of AmE, and have not studied AmE pronunciation in any depth, I am not qualified to say what the actual pronunciation of the word is for most Americans. My experience of working with Americans suggests that the LPD and the Cambridge EPD transcriptions are right, for some Americans at least, but that experience is very limited.