-である form in written Japanese


I have a question about the following example sentence:

_これは日本の会社のだ(である)。だが、 タイで作られた。_

From what I understand, である/だ and ます/です should not be mixed, and である is used
in written language.

However, is following sentence correct in a formal/written context:

_これは日本の会社のである。だが、 タイで作られた。_

Or is there an alternative to the root/た/ない form that should be used instead?
To me, it seems odd to mix -である (formal) with forms I consider casual
(た、ない、etc.)




である certainly marks the sentence as written-style, but in a style where you
are using である, the normal past (perfective) would be ～た, 作られた; similarly, the
negative would be 作らない and so on. These forms are not necessarily 'casual'.

