Can I omit the /ə/ sound?
Due to another dictionary's pronunciation [seɡˈmentl]
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Yes, you can omit the /ə/, but if you do so, you'll have to make the terminal /l/ syllabic.
As I explained in this answer, when we have a sonorant right after an obstruent in an unstressed syllable, the sonorant often tends to be syllabic i.e. it makes a syllable on its own (syllable nucleus).
The second syllable in segmental is stressed and the last syllable is unstressed. And we have a sonorant /l/ right after the /t/ (obstruent), so in this case, the /l/ is syllabic.
Syllabic consonants are often longer and more prominent than normal. In the case of segmental the air is released laterally. Normally, the stops /t/ and /d/ are released by releasing the closure from the centre of the tongue; however, in lateral release, the closure is released from the sides of the tongue.
We symbolise syllabic consonants by a small vertical line below the consonant:
Note, however, that most Americans will drop the /t/ in that position and will pronounce it something like [segˈmeɾ̃əl]1. I've explained why Americans drop the /t/'s in that position in this answer. In a nutshell, when a stressed syllable ends in an /n/ and the next unstressed syllable starts with a /t/, Americans are likely to drop the T, so 'internet' becomes innernet, 'segmental' becomes segmennal, 'counter' becomes counner etc.
(Here's how most Americans pronounce 'segmental'.)