Variable length vowels

[James Crippen]

In Tlingit there are a number of word-final vowels that seem to have
variable length, meaning that they are pronounced short by some
speakers and long by others, or even variably for the same speaker.
This is despite regular distinctions in vowel length elsewhere. I’ve
taken to transcribing them in IPA using a “half colon” /ˑ/, with the
usual triangular colon /ː/ for true long vowels.

Here are a couple of examples:

/ʔaːníˑ/ |ʔaːn-í| ‘land-pss’ = [ʔɑːnɪ́] ~ [ʔɑːní] ~ [ʔɑːníː]
/χuˑ/ |–χuˑ| ‘among (inalienable)’ = [χʊ] ~ [χu] ~ [χuː]

There are however minimal pairs of word-final vowel length:

/tʼá/ ‘king salmon (O. tschawytscha)’ = [tʼʌ́] ~ [tʼá] ~ [tʼɐ́] ~ [tʼə́]
/tʼáː/ ‘board’ = [tʼáː] ~ [tʼɑ́ː]

My question is whether or not such a phenomenon occurs in any
Athabaskan languages that list members are familiar with. Specifically
in word-final position, but perhaps anywhere. Is there any good
literature I can read on this phenomenon?

Thanks,
James